Here's an 'outside the box' technique that might work (haven't tried
it yet but maybe later):
Put a sturdy support under the shingle and press hard with a pizza
wheel as follow your chalk line. The thin circular blade should be
fairly self cleaning and if you warm the shingles well with a heat gun
the job ought to go fairly quickly. Amazon has pizza wheels if you
don't know what they look like. This would have the advantage of
avoiding cold splintering of the old shingles with toothed saw or
If you value your happy home don't try one from SWMBO's kitchen tool
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